2. If I protect the spawner with an emerald PS field to turn it on and off but that Emerald PS field is inside a diamond PS field, does the diamond field also have to have the prevent-mob-spawn toggled or does the inner emerald field override the diamond one?

As for the emerald question, I'm pretty sure that the larger field will affect whats in the smaller one so if your large field is set to disallow mob spawns it doesn't matter if the smaller one allows it, mobs still will not spawn.